Yttrium and magnesium based vanadium corrosion inhibitors

ABSTRACT

A process based on the combined use of yttrium and magnesium to inhibit vanadium corrosion of high temperature parts of thermal equipment. The combined use of yttrium and magnesium, applied in a variable yttrium/magnesium ratio, compared with conventional magnesium inhibition, may reduce emission of magnesium vanadate and minimize losses of performance due to fouling of the high temperature parts, including in the presence of alkali metals. Further, compared with inhibition based on yttrium alone, it may reduce the inhibition cost and reinforce the protection against combined vanadium pentoxide and sodium sulfate corrosion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of the French PatentApplication No. 1561782 entitled “Yttrium and Magnesium Based VanadiumCorrosion Inhibitors,” filed Dec. 3, 2015, which is incorporated byreference in its entirety.

BACKGROUND

The subject matter disclosed herein relates generally to thermalequipment, and more particularly to inhibition of high temperaturecorrosion thereof.

Thermal equipment, such as boilers, diesel engines, gas turbines,furnaces, and process reactors may be used to burn certain liquid fuels.In such equipment, the fuels may contain traces of metallic contaminantssuch as vanadium, sodium, potassium, calcium, and lead that need to betreated prior to their combustion in order to attenuate the hightemperature corrosion effects of these metals on thermal equipment. Themetallic salts contained in the fuels are water soluble and may beextracted upstream of the thermal equipment. For example, fuel washingoperations with water, followed by water/fuel separation with the aid ofelectrostatic separators or centrifuges, are commonly implemented toseparate water soluble metallic salts such as chlorides and sulfates ofsodium, potassium and partially calcium. However, the vanadiumderivatives contained in the fuels are organic in nature and are notwater soluble and therefore cannot be extracted by such a washingoperation. The presence of such organic compounds of vanadium in liquidfuels burned in the thermal equipment is likely to cause hightemperature corrosion of the metallic materials in contact with thecombustion gases.

BRIEF DESCRIPTION

Certain embodiments commensurate in scope with the originally claimedinvention are summarized below. These embodiments are not intended tolimit the scope of the claimed invention, but rather these embodimentsare intended only to provide a brief summary of possible forms of theinvention. Indeed, the invention may encompass a variety of forms thatmay be similar to or different from the embodiments set forth below.

In one embodiment, a process for inhibiting vanadium corrosion inthermal equipment includes introducing vanadium corrosion inhibitorsinto the thermal equipment. The vanadium corrosion occurs in hightemperature parts of the thermal equipment that burns a fuel containingvanadium, such that vanadium pentoxide (V₂O₅) is formed duringcombustion of the fuel. The vanadium corrosion inhibitors include anyttrium-based inhibitor comprising yttrium oxide (Y₂O₃) and amagnesium-based inhibitor comprising magnesium oxide (MgO).

In another embodiment, a process includes inhibiting vanadium corrosionin thermal equipment by conversion of vanadium pentoxide (V₂O₅) intoother chemical species using vanadium corrosion inhibitors, the V₂O₅being formed during combustion of a vanadium-containing fuel in thethermal equipment. The vanadium corrosion inhibitors inhibit vanadiumcorrosion by reducing an amount of V₂O₅ according to a reaction:V₂O₅ +yY₂O₃ +m(1−y)MgO→2yYVO₄+(1−y)Mg₃V₂O₈+(1−y)(m−3)MgO,wherein y is between about 0.05 to 0.95, and m is between about 3 to 15.

In another embodiment, a process includes inhibiting vanadium corrosionin thermal equipment using vanadium corrosion inhibitors, the thermalequipment being used to burn vanadium-containing fuels. The vanadiumcorrosion inhibitors include an yttrium-based and a magnesium-basedvanadium corrosion inhibitor. The inhibiting of the vanadium corrosionis performed such that vanadium pentoxide (V₂O₅), vanadium oxide, andyttrium sulfate are not released from the thermal equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a plot illustrating a comparison between different turbineperformance levels as a function of time, in accordance with certainembodiments of the disclosure.

DETAILED DESCRIPTION

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, all features of an actual implementation may not bedescribed in the specification. It should be appreciated that in thedevelopment of any such actual implementation, as in any engineering ordesign project, numerous implementation-specific decisions must be madeto achieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

When introducing elements of various embodiments of the presentinvention, the articles “a,” “an,” “the,” and “said” are intended tomean that there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

Embodiments of the present invention concern the inhibition of hightemperature corrosion of materials of thermal equipment such as boilers,diesel engines, gas turbines, furnaces and process reactors, which, intheir furnace, burn vanadium-contaminated fuels. In particular,so-called heavy or residual fuels and crude oils, which in this documentwill be designated with the generic term “fuels,” generally containtraces of metallic contaminants such as vanadium, sodium, potassium,calcium and lead that need to be treated prior to their combustion inorder to attenuate the high temperature corrosion effects of thesemetals. In what follows, gas turbines or “turbines,” which generallyinclude: (i) a compressor; (ii) a furnace, which itself includes of aset of “combustion chambers” and (iii) a turboexpander, will be taken asparadigms of thermal equipment, although all the considerationscontained in this document apply to thermal equipment in general. The“flame temperature” of a gas turbine, which largely determines itsefficiency, is the temperature that prevails at the inlet of theturboexpander and not that which is present in the flames, which exceeds2000° C. on the flame front.

The metallic salts contained in the fuels can, when they are watersoluble, be extracted upstream of the thermal equipment; this is how“fuel washing” operations with water, followed by water/fuel separationwith the aid of electrostatic separators or centrifuges, are commonlyimplemented to separate water soluble metallic salts such as chloridesand sulfates of sodium, potassium and partially calcium.

The vanadium derivatives contained in the fuels are organic in natureand have as major drawback that they are not water soluble but fatsoluble and therefore cannot be extracted by such a washing operation.The presence of such organic compounds of vanadium in liquid fuelsburned in thermal equipment is likely to cause high temperaturecorrosion of the metallic materials in contact with the combustiongases. In fact, depending on the oxidation-reduction conditions thatprevail in the flames, the vanadium reacts with oxygen to form one ofthe oxides VO, V₂O₃, V₂O₄ (or VO₂) or V₂O₅: whereas the first threeoxides are refractory, with melting points in excess of 1500° C.,vanadium pentoxide V₂O₅, which is formed in the highly oxidizing flamesin gas turbines in particular, melts at a temperature of 670° C. Thisoxide is therefore present in liquid form in the operating conditions ofthe turbine and the fraction that is deposited on the surfaces of thehot parts may thereby cause electrochemical type corrosion in a moltensalt environment. This “vanadium corrosion” may be more or less severedepending on the nature of the metal or the alloy of the thermalequipment, the operating temperature range, and the operating time andconditions. In addition, such corrosion aggravated and more difficult toprevent when the fuel contains also alkali metals (sodium; potassium).

The corrosion potential of vanadium pentoxide V₂O₅ can be inhibited by“trapping” the latter within the refractory compounds with the aid ofchemical compounds called “inhibitors.” The classical representatives ofthese inhibitors are alkaline earth compounds such as calcium oxide,when the fuel does not contain sulfur, or magnesium salts, which can beapplied in water soluble or fat soluble form. Such magnesium-basedinhibitor additives, once introduced into a flame, decompose there intomagnesium oxide (MgO), which reacts with V₂O₅ to form magnesiumvanadate. A sufficient quantity of magnesium is introduced to generatemagnesium orthovanadate, formula Mg₃V₂O₈, whose high melting point(1070° C.) allows the vanadium charged particles to pass through the“hot gas path” of the turbine in solid form without causing corrosion ofthe hot parts of the said turbine. The inhibitor dosage must besufficient to both allow all of the vanadium present in the fuel to betrapped and to prevent the formation of vanadates having lower Mg/Vstoichiometric ratios, notably pyrovanadate (Mg₂V₂O₇) or metavanadate(MgV₂O₆) which are not refractory enough to achieve the intendedinhibition effect.

This magnesium inhibition leads to the formation of “magnesium-vanadium”combustion ashes having high melting points and containing:

-   -   on the one hand, magnesium orthovanadate (Mg₃V₂O₈) which is        produced by the following reaction (1):        V₂O₅+3MgO→Mg₃V₂O₈  (1)    -   on the other hand, an excess of magnesium in the form of        magnesium oxide (MgO) that is partly converted into magnesium        sulfate (MgSO₄).

Indeed, in the furnace, the sulfur contained in the fuel is oxidizedinto sulfur oxides (SO_(x), or SO₂+SO₃), which also react with themagnesium oxide MgO to form magnesium sulfate (MgSO₄). Therefore, to acertain extent, there is competition between V₂O₅ and SO_(x), the sulfuroxides, in the reaction with MgO. This “parasitic” reaction that formsmagnesium sulfate means that in order to trap all the vanadium, a highexcess of magnesium with respect to the stoichiometry of reaction (1)has to be added, which, in practice, results in a magnesium/vanadiumratio greater than or equal to 3 in weight. This high excess ofmagnesium is also useful, not only from a theoretical viewpoint, toensure the conversion of vanadium into magnesium orthovanadate, but alsofrom a practical viewpoint, to overcome possible inaccuracies or errorsrelated to the in-service determination of the vanadium content of thefuel.

The magnesium inhibition treatment can be characterized by the dosingratio MgO/V₂O₅ expressed on a molar basis, which will be designated as“m.” This dosing ratio “m” is taken to be equal to 12.6 in the“conventional inhibition” process and is equivalent to the Mg/V ratio of3 on the previously mentioned weight basis. The corresponding materialbalance equation is therefore, in practice, written as the followingequation (2):V₂O₅+12.6MgO+9.6sSO₃→Mg₃V2O₈+9.6sMgSO₄+9.6(1−s)MgO  (2)In this equation, “s” designates the proportion of the excess ofmagnesium that is converted into sulfate and (1−s) that which isconverted into oxide, the number s decreasing with temperature.

In the more general case of magnesium dosage, the material balanceequation is written as the following equation (3):V₂O₅ +mMgO+(m−3)sSO₃→Mg₃V₂O₈+(m−3)[sMgSO₄+(1−s)MgO]  (3)In this equation, m≥3, where m=12.6 in the case of conventionalinhibition. If abstraction is made of the magnesium sulfatation process,balance equations (2) and (3) are equivalent to the following equations(2b) and (3b) respectively:V₂O₅+12.6MgO→Mg₃V₂O₈+9.6MgO  (2b)V₂O₅ +mMgO→Mg₃V₂O₈+(m−3)MgO  (3b)The number (m−3) in the right member of equation (3b) represents therelative excess of magnesium, that is to say the excess of magnesium inrelation to one mole of V₂O₅; it is equal to 9.6 in the case ofconventional inhibition.

The main advantage of magnesium inhibition resides in its low cost. Itsmain drawback resides in the fact that the magnesium-vanadium asheswhich are formed by the mineral mix in the right member of equation (3)have a tendency to deposit significantly on the hot parts of the turbineand therefore to foul the latter, thereby gradually decreasing theenergy performance of the said turbine. In fact, a substantial fractionof the magnesium-vanadium ashes resulting from this inhibition processdeposits itself on the walls of the combustion chambers and on thethermal equipment components located downstream of the latter: this willbe referred to as an ash deposition process. This process causesprogressive fouling of the thermal equipment as it is operated andentails a correlative and progressive loss of its energy performance(power output and efficiency).

To remedy this unwanted effect, either dry cleaning or water washing ofthese ash deposits is conventionally carried out. Dry cleaning includesintroducing into the operating equipment a slightly abrasive material,free of corrosive or ash generating compounds, so as to remove part ofthe deposits accumulated on the walls of the hot parts. Water washing ofthe turbine is another, more effective, method to restore performanceand includes injecting hot water after shutdown and cool down of theturbine, thereby dissolving the water soluble fraction of themagnesium-vanadium ashes, that is to say, the magnesium sulfate, whichresults in the destabilization of the entire ash layer and allows thealmost complete entrainment of the said deposit; this method thereforeallows the initial performance levels to be restored almost completely.

In conclusion, because of their relatively fouling nature,magnesium-vanadium ashes, in addition to the previously mentioned lossesof power and efficiency, require frequent shutdowns of the turbine tocarry out such water washes, thereby reducing the availability of theproduction facility. In what follows, these losses of efficiency, powerand availability will collectively be designated by the expression “lossof performance by fouling.”

A second significant drawback of magnesium inhibition is that it issensitive to the presence of alkali metals that tend to reduce itseffectiveness due to the formation of mixed magnesium and for examplesodium vanadates, in particular NaMg₄(VO₄)₃, which melts at atemperature above 750° C. and therefore lowers the melting point of themagnesium-vanadium ashes, making them more adhesive. In the descriptionthat follows, this negative effect of alkali metals on the fusibility ofthe magnesium-vanadium ashes will be designated as the “adverse effectof alkali metals.”

A third significant drawback is that at high temperaturesmagnesium-vanadium deposits become highly fouling which is due, on theone hand, to the partial or complete melting of Mg₃V₂O₈ (if thetemperature locally exceeds 1070° C.) and, on the other hand, to thedecomposition of the magnesium sulfate into magnesium oxide (“s” tendingtoward 0 in equations (2) or (3) above), which oxide, being totallyinsoluble in water, cannot be eliminated by the washing operation. Itfollows that when the “flame temperature” of the turbine exceeds a limiton the order of 1090° C., the effectiveness of the washing operationbecomes very low and restoration of the performance levels problematic:this temperature of 1090° C. therefore constitutes the “technical limit”of operation of turbines burning vanadium contaminated fuels.

As an alternative to magnesium inhibition, the use of yttrium-basedinhibitors has been suggested. U.S. Pat. No. 5,637,118 in fact suggestssuch a vanadium corrosion inhibitor which is also claimed to beeffective against combined vanadium pentoxide and sodium sulfatecorrosion. This document suggests the use of yttrium in the form of acompound which at high temperatures generates yttrium oxide (Y₂O₃) whichcombines with V₂O₅ to form yttrium vanadate (YVO₄), which is highlyrefractory, with a melting point in excess of 1800° C. and thereforeremains solid at the operating temperatures of the gas turbine. Thecomposition of such yttrium-based additives includes at least astoichiometric amount of yttrium ester having at least four carbon atomsand a hydrocarbon fuel soluble chelating agent that includes2,4-pentanediene.

The reaction for stoichiometric inhibition of vanadium by yttrium istherefore written as the following equation (4):V₂O₅+Y₂O₃→2YVO₄  (4)The inventors of the present application have empirically observed thatthis yttrium-based inhibition has the major advantage that it generatesonly very small ash deposits on the hot parts of thermal equipment andtherefore substantially reduces losses of performance due to foulinginherent in magnesium inhibition. Yttrium also has the advantage that itis a very powerful inhibitor, reaction (4) always prevails over thereaction forming the sulfate Y₂(SO₄)₃, and as such requires, in theory,only a mixing ratio Y₂O₃/V₂O₅ equal to or slightly greater than thestoichiometric ratio, which, according to reaction (4), is equal to 1 onan atomic basis. However, in practice and as in magnesium inhibition,this ratio needs to be significantly higher than 1 to overcome possibleinaccuracies or errors relating to the in-service determination of thevanadium content in the fuel. Thus, the aforementioned U.S. Pat. No.5,637,118 recommends a preferred atomic mixing ratio y=Y₂O₃/V₂O₅, whichis at least equal to 1.25 on an atomic base, the relative excess ofyttrium, which is defined as the number (y−1), being then greater thanor equal to 0.25.

In these conditions, taking into account the strong affinity of yttriumoxide (Y₂O₃) for sulfur trioxide, this excess of yttrium generatesyttrium sulfate Y₂(SO₄)₃ such that the material balance equation of theinhibition with, for example, an atomic Y/V ratio of 1.25, is written asthe following equation (5):V₂O₅+1.25Y₂O₃+0.25s′SO₃→2YVO₄+0.25s′Y₂(SO₄)₃  (5)In this equation, s′ designates the proportion of the excess of yttriumthat is converted into sulfate; s′ decreases only very little withtemperature, remaining substantially equal to 1 over the temperaturerange of the inhibition application.

The corrosion potential of vanadium pentoxide V₂O₅ can be inhibited bytrapping the latter within the refractory compounds with the aid ofchemical compounds called inhibitors. In the case of any excess ofyttrium, the material balance equation of the inhibition will thereforebe written as the following equation (6):V₂O₅ +yY₂O₃+(y−1)s′SO₃→2YVO₄+(y−1)s′Y₂(SO₄)₃  (6)The number represented by y is at least equal to 1 and relative excessof yttrium is (y−1). It is important to note that when y is less than 1,which corresponds to incomplete inhibition, no yttrium sulfate is formedbecause chemically the formation of vanadate prevails over that ofsulfate.

If abstraction is made of the yttrium sulfatation process, this materialbalance equation (6) is equivalent to the following simple equation(6b):V₂O₅ +yY₂O₃→2YVO₄+(y−1)Y₂O₃  (6b)It should be noted that the choice of the relative excess (y−1) of 0.25recommended by U.S. Pat. No. 5,637,118 (reaction (5) versus reaction(4)) is, however, by no means conservative, as this relative excess is9.6 in the case of conventional magnesium inhibition.

A second advantage of yttrium inhibition, put forward in U.S. Pat. No.5,637,118, is the fact that it is insensitive to the presence of alkalimetals, because, according to the same patent, it is effective againstcombined vanadium pentoxide and sodium sulfate corrosion. However, thedrawback of yttrium, as a rare earth metal, is its high cost. Oncereleased into the ambient air, vanadium components, like the majority ofthe so-called “heavy” metals, and in particular vanadium pentoxide V₂O₅which is an acid, reactive and “mobile” oxide, do not have a neutralimpact on the environment. Installations that burn “fuels” are dependenton this situation which is actually independent of the thermal equipmentused, as it results directly from the use of such unrefined fuels: theconversion of V₂O₅ into alkaline earth vanadate, which results from themagnesium-based vanadium inhibition treatment, tends to reduce thisimpact because the vanadium is then immobilized by a light metal (Mg)with high basic power, within the magnesium orthovanadate which is muchless reactive than V₂O₅ but can nevertheless be subject to leachingprocesses in aqueous or humid environments. Consequently, thecombination of magnesium with vanadium has an attenuating effect on theimpact of V₂O₅ on the environment, and the magnesium oxide and sulfatepresent in excess (reaction (3)) are moreover environmentally benigncompounds.

The case of yttrium inhibition appears a priori to be more complexbecause although this element too has a high basic power capable ofimmobilizing vanadium, its use, necessarily in excess and according toreaction (6), leads to the formation of yttrium sulfate which is watersoluble and may therefore contribute to the diffusion of yttrium in ahumid or aqueous environment. By contrast, yttrium vanadate YVO₄ notonly has a very high melting temperature (1810° C.) but is alsoextremely stable: this highly refractory and inert character enables itto be “immobilized” in solid form as soon as the flame goes out. Anessential fact as observed by the inventors of this application, is thatit is strictly insoluble in water. Biotoxicity tests carried out by theinventors of this application in accordance with standard OCDE 209relating to plants, have shown that yttrium vanadate YVO₄ affectsneither the germination or the growth of plants. This a priorisurprising fact, because it involves the combination of two so-calledheavy metals (vanadium and yttrium), could possibly be explained by thefact that the vanadium and the yttrium are solidly “immobilized one bythe other” in the molecule YVO₄, which is insoluble in water andchemically inert, is therefore not reactive in biological processes.

Moreover, the inventors of this application have also empiricallyobserved that when an yttrium compound and a magnesium compound areplaced in the presence of vanadium oxide, yttrium vanadate is alwaysformed preferentially to magnesium vanadate. Consequently, if oneintroduces, on the one hand, yttrium in default with respect to thevanadium—which is equivalent to using a value of y less than 1 inequation (6b)—and, on the other hand, sufficient magnesium to inhibit,according to reaction (3b), the vanadium fraction that is not inhibiteddue to the insufficient yttrium, then YVO₄, with the exclusion ofY₂(SO₄)₃ on the one hand, and classical products of themagnesium-vanadium inhibition, notably (MgSO₄+MgO) and Mg₃V₂O₈ will beformed, which, as previously indicated, has an attenuating effect on theimpact of V₂O₅ on the aqueous environment.

Thus, the inventors of the present application have discovered theinterest in using a vanadium inhibition method based on the combined usein variable ratios of:

-   -   on the one hand, yttrium used in default with respect to the        vanadium, yttrium being an expensive metal but capable of        minimizing losses of performance due to fouling of the hot parts        and moreover free of environmental impact by aqueous media        because it is used in default    -   on the other hand, magnesium, a less expensive metal than        yttrium and attenuating the environmental effects of V₂O₅, but        causing significant losses of performance due to fouling of the        hot parts.

In this novel so-called “combined inhibition” method, for each mole ofV₂O₅ formed during combustion, the following are introduced:

-   -   a number y, strictly positive and strictly less than 1, of moles        of yttrium oxide that inhibit y moles of V₂O₅ according to the        following balance equation (7) derived from equation (6b) where        the value of y is chosen to be between 0 and 1:        V₂O₅ +yY₂O₃→2YVO₄+(y−1)Y₂O₅  (7)    -   a number represented by m(1−y) of moles of magnesium, m being        greater than or equal to 3 thereby totally inhibiting the        residual V₂O₅ which is the vanadium pentoxide not inhibited by        the yttrium; the inhibition balance of this residual V₂O₅, which        derives from equation (3b) is written as the following equation        (8):        (1−y)V₂O₅ +m(1−y)MgO→(1−y)Mg₃V₂O₈+(1−y)(m−3)MgO  (8)

The overall balance of this “combined yttrium and magnesium inhibition”is the complete inhibition of the vanadium, with excess magnesium, whichcan therefore be expressed by the following overall equation (9):V₂O₅ +yY₂O₃ +m(1−y)MgO→2yYVO₄+(1−y)Mg₃V₂O₈+(1−y)(m−3)MgO.   (9)This equation reveals y and m as process variables. The choice of theparameter m is arbitrary, it being understood, however, that the choiceof a value of m greater than 15, in addition to the increased costrelated to the magnesium consumption, would make the reduction in“losses of performance by fouling” very difficult. The parameter “m”will therefore preferably vary from 3 to 15 and can be equal to 12.6.For example, if one keeps the value of the conventional magnesiuminhibition, the relative excess (m−3) is then equal to 9.6 and theatomic Mg/Y ratio, written as 0.5 m(1−y)/y, then depends only the valueof y.

The vanadium corrosion inhibition process according to the presentdisclosure therefore uses two vanadium inhibitors—yttrium andmagnesium—whose combination allows, in addition to effective inhibitionof the said vanadium corrosion:

-   -   to control the environmental impact of the ashes to the extent        that neither vanadium pentoxide V₂O₅, nor oxide, nor yttrium        sulfate and limited quantities of magnesium orthovanadate        Mg₃V₂O₈, which is the sole product capable of diffusing into the        environment through wet deposition, are formed;    -   to control the ash deposition, which becomes lower as the        parameter y is close to unity (i.e., close to 1).

This possibility to control the ash deposition is particularlyinteresting. In fact, it is possible:

-   -   on the one hand, to increase the flame temperature beyond the        “technical limit” on the order of 1090° C., which has been        previously defined without encountering any major problem of        eliminating the ash deposits during the turbine water washes;    -   on the other hand, based on the experience acquired during the        operation of the thermal equipment, to determine:        -   either the minimum value y₁ of the parameter y to ensure            that the operating time between two consecutive washing            operations does not fall below a predetermined value T,            while at the same time complying with a maximum authorized            loss of performance: this operating criterion will be            designed as the minimum operating time between two washes            criterion;        -   or the minimum value y₂ of the same parameter y to ensure            that the performance loss rate (dP/dt) by fouling does not            exceed, in absolute value, a predetermined threshold δ: this            operating criterion will be designed as the performance loss            minimization criterion.

FIG. 1 illustrates this double point. In this FIGURE, the turbineperformance P, which is either the power output (in kW/h), or theefficiency (in %), is a function of time t (in hours). The performancelevel (3) is the original performance level of this turbine. Theperformance level (4) is the threshold from which the operator decidesto proceed with a washing operation, and is, for example, taken to beequal to 95% of the original performance (3), it being understood thatthe operator may also choose to wash the turbine after a defined andinvariable number of operating hours, which is a less common strategy,as is the case in example 2 below. The time intervals (5), (6), (7) and(8) represent four turbine operating periods for which four differentchoices for the parameter y are made. These periods are inserted betweenthe washing operations (9) to (13) during which the power output (14) to(18) is zero, but at the end of which the turbine again has aperformance level (19) to (23) identical to the original level (3).

During the operating period (6), the value adopted for y is equal to y₁,which is the value that ensures an operating period between two washesthat is equal to the lower limit T, predetermined by the operator,without exceeding a loss of performance of 5%. In other words, thisoperating period (6) just satisfies the previously defined “minimumoperating time between two washes criterion.” During the operating time(5), the value adopted for y is less than y₁ so that the duration ofthis operating period (5) is shorter than T: this operating period (5)does not satisfy the minimum operating time between two washescriterion. During the operating time (7), the value adopted for y isequal to y₂, this value ensures, in absolute value, a performance lossrate, represented by the slope (24) that is equal to the upper limit δpredetermined by the operator. In other words, this operating period (7)just satisfies the performance loss minimization criterion. Finally,during the operating time (8), the value adopted for y is greater thany₂ so that the performance loss rate over this period (8), which isrepresented by the slope (25), is less than the performance loss rate δpredetermined by the operator and therefore satisfies, with a certainmargin, the performance loss criterion.

The operator may, in a prior experience acquisition phase, determine thevalues of y₁ and y₂ by following, for a given fuel grade, theperformance of its machine as a function of the parameter y of thecombined inhibition. The above-described FIG. 1 has been included inthis description for illustration purposes only and does not cover allof the Y and Mg mixtures that are made possible by the presentembodiment and that depend on parameters y and m: for example, thefollowing cases could have been envisaged: y>y₁; y<y₂; y₁<y<y₂;3<m<12.6; etc. The combined inhibition process according to the presentdisclosure can therefore be used in multiple implementation methods thatresult from the choice of the parameters y and m on the one hand, andfrom the methods of introducing the two inhibitors on the other hand.

Finally, it should be noted that the inventors of this application havealso observed that the combined use of magnesium and yttrium improves,as compared to yttrium alone, the quality of protection against combinedvanadium pentoxide and sodium sulfate corrosion, the said protectionbeing already an advantage claimed by the process described in U.S. Pat.No. 5,637,118. This result could possibly be explained by the formationof a mixed sodium/magnesium sulfate where the sodium sulfate is blockedaccording to the following reaction (10):Na₂SO₄+3MgSO₄→Na₂Mg₃(SO₄)₄  (10)

Thus, the present disclosure concerns a process for inhibiting hightemperature corrosion of the hot parts of thermal equipment that burns avanadium-contaminated fuel in the presence or absence of sodium. Thisprocess is characterized in that, in a furnace of the said thermalequipment, the following are introduced per mole of vanadium pentoxideformed during combustion:

-   -   a number y, varying from 0.05 to 0.95, of mole of yttrium oxide,    -   and a number of moles of magnesium oxide equal to m*(1−y), m        varying from 3 to 15 and preferably being equal to 12.6, the        inhibition of the vanadium taking place according to the        equation (9):        V₂O₅ +yY₂O₃ +m(1−y)MgO→2yYVO₄+(1−y)Mg₃V₂O₈+(1−y)(m−3)MgO,   (9)        where y and m are those as defined above.

The inhibition of the vanadium is then complete because it is carriedout with an excess of magnesium, according to the overall reactionfollowing the equation (9). It is understood that although the category“oxide” is used to define the yttrium and magnesium compounds to beintroduced, one can also introduce any other yttrium (or magnesium)compound that is capable of generating yttrium oxide (or magnesiumoxide) in the furnace. It is also understood that the expression “isintroduced into the furnace” does not necessarily mean direct injectioninto the furnace, the introduction can also take place upstream of thefurnace in any circuit that terminates at the said furnace.

This combined inhibition method has the advantages that with respect toyttrium inhibition as described in U.S. Pat. No. 5,637,118, theinhibition cost is reduced and the protection against combined vanadiumpentoxide and sodium sulfate corrosion is reinforced. Further, withrespect to conventional magnesium inhibition,

-   -   the emission of leachable magnesium orthovanadate is reduced,    -   the ash deposition on the hot parts of the thermal equipment and        the correlative losses of performance are reduced,    -   the adverse interference of alkali metals that may be present in        the fuel arriving in the furnace is reduced: the loss of        performance by fouling of the hot parts, also in the presence of        alkali metals, is thus minimized,    -   the flame temperature is increased beyond the technical limit on        the order of 1090° C.

In one embodiment, the fuel is also contaminated with sodium. In oneembodiment, the thermal equipment is a gas turbine whose flametemperature is less than, equal to or greater than 1090° C. In oneembodiment, two separate inhibitors based on yttrium and magnesiumprecursors respectively can be used. For example, the yttrium oxide isgenerated from a fat soluble or water soluble precursor, the saidprecursor being contained in an additive called yttrium-based inhibitor.For example, the magnesium oxide is generated from a fat soluble orwater soluble precursor, the said precursor being contained in anadditive called magnesium-based inhibitor.

In another embodiment, an inhibitor combining these two precursors canbe used, the additives concerned may be water soluble or fat soluble. Inanother embodiment:

-   -   the precursor of the yttrium oxide is preferably yttrium        nitrate, an yttrium sulfonate, an yttrium carboxylate, an        yttrium chloride, or a nanometric yttrium compound in suspension        in a hydrophilic or lipophilic solvent, and    -   the precursor of the magnesium oxide is an inorganic magnesium        salt, a magnesium sulfonate, a magnesium carboxylate, or a        nanometric magnesium compound in suspension in a hydrophilic or        lipophilic solvent.

In another embodiment, one of the two inhibitors is introduced directlyinto the fuel or into the furnace or, upstream of the furnace, into anyof the supply circuits of the furnace of the thermal equipment, and theother inhibitor is introduced at a location different from the firstinhibitor, for example by mixing it into the fuel, for example in amixing, storage or recirculation tank before sending it into any of thesupply circuits of the furnace of the thermal equipment. In anotherembodiment:

-   -   either both inhibitors are introduced directly into the fuel or        into the furnace or upstream of the furnace into one of the        supply circuits of the furnace;    -   or they are mixed directly with the fuel, for example in a        mixing tank before they are circulated through any of the        circuits terminating at the furnace of the thermal equipment.

Supply circuits of the thermal equipment furnace are understood to mean,for example, a fuel supply circuit, or a water supply circuit, or awater/fuel oil emulsion circuit, or an atomizing air circuit.Furthermore, direct injection into the furnace is understood to meanthat for example an inlet provided for direct injection into the furnaceof either a cleaning product or a combustion improvement product, isused. For example, in a gas turbine combustion chamber, particles areinjected to clean the first turbine stages. Thus, according to oneembodiment, the injection of any of these two inhibitors is carried out:

-   -   either upstream of the furnace and via a fuel supply circuit or        a water supply circuit or an atomizing air circuit of the said        furnace,    -   or directly into the furnace, in particular via a nozzle used        for the injection of particles designed for cleaning the turbine        or via a dedicated nozzle.

In another embodiment, the parameter y is chosen to be at least equal to0.9. In this case, the performance losses due to fouling and theemission of magnesium vanadate particles are minimized. In anotherembodiment, the parameter y is chosen to be at most equal to 0.1. Inthis case, the inhibition cost is minimized. The balance equation (9) ofthe inhibition is then written, by taking y=(1−e), as the equation (11):V₂O₅+(1−e)Y₂O₃ +meMgO→2(1−e)YVO₄ +eMg₃V₂O₈ +e(m−3)MgO,  (11)the number represent by e having a low value, for example equal to 0.1.In another embodiment, the parameter y is chosen to be greater thanvalue y₁ so as to obtain a power loss rate for the thermal equipmentthat is less than a predetermined limit. In another embodiment, theparameter y is chosen to be greater than value y₂ so as to obtain anoperating time between two consecutive washes of the turbine that isgreater than a predetermined limit.

EXAMPLES Example 1

A highly contaminated fuel containing 150 mg vanadium (V) per kilogram(or 150/50.9=2.97 milliatoms of V per kg fuel) is burned in a gasturbine whose flame temperature is 1088° C. (therefore practically atthe 1090° C. technical limit level) and which has a power output of 101MWe (e.g., electrical MW). The criterion chosen by the operator foractivation of a turbine wash is a 5% loss in power due to fouling, sothat the operator shuts down the turbine when it has lost 0.05*101=5.05MW and performance has therefore fallen to 101−5.05 or approximately 96MW. Initially, he also applies conventional magnesium inhibition whichrequires the injection of 150*3=450 mg Mg per kg fuel and is carried outaccording to the following balance equation (2b):V₂O₅+12.6MgO→Mg₃V₂O₈+9.6MgO  (2b)The emission of magnesium orthovanadate (PM=302.7), calculated on thebasis of the number of vanadium atoms, is equal to:[(150/50.9)/2]*302.7=446 mg of Mg₃V₂O₈ per kg burned fuel.

Furthermore, the operator observes that the loss of power due to foulingof the turbine takes place at an average rate of 51 kWe per operatinghour, which results in an operating time of 5.05/0.051 or approximately100 hours between two consecutive washes. The operator decides to switchto a combined yttrium/magnesium inhibition method according to theprocess in accordance with the present embodiment, because he wishes, onthe one hand, to reduce the emission of magnesium-vanadium particlesand, on the other hand, to increase the availability of his machine byreducing by a factor of 3 the power loss rate due to fouling: hetherefore targets a power loss rate on the order of 51/3=17 kW/h.

Based on previously conducted tests during which he monitored the lossof power as a function of the parameter y of the combined inhibitionmethod, the operator has determined that the value y₁ to be used toreach this objective is on the order 0.93 by taking a value of 12.6 forthe parameter m, which, according to equation (9), yields the followinginhibition balance equation (12):V₂O₅ +yY₂O₃+m(0.882−y)MgO→2yYVO₄+(0.07−y)Mg₃V₂O₈+(0.672−y)(m−3)MgO.  (12)In this combined inhibition, the atomic weight of yttrium being 88.9g/mole, the operator has to inject:

-   -   yttrium: 2.97*0.93*88.9=245.6 mg of Y per kg of fuel.    -   magnesium: 450/12.6*0.88=31.4 mg of Mg per kg of fuel.        This change in inhibition method decreases the emission of        magnesium orthovanadate from 446 mg to 446*0.07=31.2 mg Mg₃V₂O₈        per kg burned fuel: it is therefore divided by a factor of 14.        Simultaneously, the performance loss rate effectively falls from        51 kW/h to around 16.5 kW/h. As a result, the operating time        possible between two washes increases from 100 hours to        5.05/0.0165=306 hours.

The operator has therefore succeeded in tripling the operating time ofits turbine between two washes. Since a complete washing operation,which includes shutdown, cool-down, water washing, spinning, and restartof the turbine, takes around 15 hours, the availability of the gasturbine which was 100/(100+15)=90% with conventional magnesiuminhibition, becomes: 306/(306+15)=95% with yttrium inhibition. Theoperator thus sees the availability of its turbine increase by 5%, whilethe emission of magnesium orthovanadate is divided by 14. Asmagnesium/yttrium based inhibitors, the operator uses an aqueoussolution containing 2% magnesium in the form of magnesium nitrate and15.6% yttrium in the form of yttrium nitrate, a solution that isdirectly injected into the low pressure section of the fuel circuit, viaa high speed rotary mixer that ensures the emulsification of thissolution in the fuel. Taking into account the high quantities ofvanadium to be inhibited, this option of water soluble inhibitors iseconomically more interesting than the option of fat soluble inhibitors.

Example 2

A lowly contaminated fuel containing 30 mg V per kilogram (or30/50.9=0.589 milliatom of V per kg fuel), is burned in a gas turbinewhose flame temperature is also 1088° C. (therefore practically at the1090° C. technical limit level) and which has a power output of 38 MWe(e.g., electrical MW).

Initially, the operator also applies conventional magnesium inhibitionwhich requires the injection of 30*3=90 mg Mg per kg fuel and is carriedout according to the following balance equation:V₂O₅+12.6MgO→Mg₃V₂O₈+9.6MgO  (2b)This time, the operator does not wish to increase the availability ofits turbine because its operating strategy includes activating a turbinewash every 6 days (i.e., every 145 hours), as it is more economical tocarry out these washes on Sundays, when demand for electricity is low.Instead, the operator wishes to increase the productivity of itsfacility, with a 5% gain in electricity production as target. Withconventional magnesium inhibition, the loss of power due to fouling ofthe turbine takes place at an average rate of 21 kWe per operating hour.After 145 operating hours, the turbine has therefore lost0.021*145=3.045 MWe and performance has fallen to 38−3.045=34.96 MWe.Consequently, the average power output over the 145 hour operating timebetween two washes is (38+34.96)/2=36.48 MWe and the quantity ofelectricity generated is 145*36.48=5290 MWh.

Based on previously conducted tests during which he monitored the lossof power as a function of the parameter y of the combined inhibitionmethod, the operator has determined that the value y₂ to be used toreach this objective is on the order 0.75 by taking a value of 12.6 forthe parameter m, which, according to equation (9), yields the inhibitionbalance equation (13):V₂O₅+0.75Y₂O₃+3.15MgO→1.5YVO₄+0.25Mg₃V₂O₈+2.4MgO.  (13)In this combined inhibition according to the process in accordance withthe present embodiment, the operator must therefore inject:

-   -   yttrium: 0.589*0.75*88.9=39.2 mg of Y per kg of fuel.    -   magnesium: 90/12.6*3.15=22.5 mg of Mg per kg of fuel.

The operator establishes that the power loss rate falls from 21 kW/h toapproximately 10.05 kW/h. After 145 hours, the turbine has thereforelost 0.0105*145=1.523 MWe and performance has therefore fallen to38−1.523=36.477 MWe. Consequently, the average power output over the 145hour operating time between two washes is (38+36.477)/2=37.24 MWe. Thus,over an operating time between two washes, the operator who wished toincrease his production, wins on average 37.24−34.96=2.28 MWe: over thistime he therefore generates 2.28*145=330 MWh more electricity. The gainobtained by switching to combined yttrium/magnesium inhibition accordingto the process in accordance with the present embodiment, amounts to330/5290=6.2%. He has therefore slightly exceeded the targeted 5% gain.

One can furthermore determine that this change in inhibition methodcauses the emission of magnesium orthovanadate to fall from 446 mg to112 mg Mg₃V₂O₈ per kg burned fuel: it is therefore divided by a factorof approximately 4. The operator uses separate magnesium and yttriuminhibitors, both of the fat soluble type: the magnesium-based inhibitoris a magnesium sulfonate solution with 11% magnesium in a heavy aromaticnaphtha and the yttrium-based inhibitor is an yttrium octoate solutionwith 5% yttrium, also in a heavy aromatic naphtha. These inhibitors areinjected in online mode into the low-pressure section of the fuelcircuit with the aid of two distinct dosing pumps, but at the same pointwhere a single static mixer is installed to ensure their mixing with thefuel.

Example 3

The turbine and the fuel that it burns are identical to those in example2 above. The operator who initially uses conventional magnesiuminhibition, has at the turbine inlet a fuel that is free of alkalimetals thanks to water washing of the fuel carried out by a fueltreatment system based on centrifugal separators that ensures a residualsodium content of less than 1 ppm in weight at the inlet of the turbine,thereby preventing the adverse effect of metals on fouling of theturbine. He observes an inadvertent malfunction of his fuel treatmentsystem resulting in a residual sodium content on the order of 10 ppm.Correlatively, he observes that the power loss rate due to foulingincreases from 21 kWe to 30 kWe, reducing the production and theefficiency of his machine. He then decides to switch to a combinedyttrium/magnesium inhibition method in accordance with the presentdisclosure, with the same parameter y as in example 2, i.e., y=0.75.After switching to this combined inhibition method and before he hasresolved the malfunction of his fuel treatment system that results in ahigh sodium content at the turbine inlet, the operator observes, overtwo consecutive operating times of 100 hours, that the power loss rateis only 12 kW/h, or a reduction by a factor of 30/12=2.5 of theperformance loss incurred when conventional magnesium inhibition wasapplied.

Example 4

The turbine and the fuel that it burns are identical to those used inexample 2 above. The operator initially applies conventional magnesiuminhibition and again wishes to increase the productivity of his facilityby increasing the efficiency of his turbine, for which purpose heenvisages to switch to a flame temperature of 1140° C., whichcorresponds to a 47° C. increase with respect to the initial value of1088° C. For this, he decides to switch to combined yttrium/magnesiuminhibition according to the process in accordance with the presentdisclosure, with a value of y of 0.95 and value 12.6 for the parameterm. The corresponding inhibition equation (14) is as follows:V₂O₅+0.95Y₂O₃+0.63MgO→1.9YVO₄+0.05Mg₃V₂O₈+0.48MgO  (14)Over several wash cycles, the operator observes that:

-   -   the initial power (machine washed) increases from 100 MWe to 112        MWe,    -   the power loss rate remains at a level of 15 kW per hour.        Moreover, the emission of orthovanadate falls from 892 mg to        892*0.05=45 mg per kg burned fuel.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

The invention claimed is:
 1. A process for inhibiting vanadium-inducedcorrosion in high temperature parts of thermal equipment duringoccasions when the thermal equipment combusts a fuel containingvanadium, the process comprising: introducing vanadium corrosioninhibitors into a fluid flow path of the thermal equipment, whereincombustion of the fuel containing vanadium in the thermal equipmentresults in the formation of vanadium pentoxide (V₂O₅); and reducing anamount of V₂O₅ by causing a reaction between V₂O₅ and the vanadiumcorrosion inhibitors; wherein the vanadium corrosion inhibitors comprisean yttrium-based inhibitor comprising yttrium oxide (Y₂O₃) and amagnesium-based inhibitor comprising magnesium oxide (MgO).
 2. Theprocess of claim 1, wherein the reducing of the amount of V₂O₅ by thereaction of V₂O₅ with the vanadium corrosion inhibitors inhibitsvanadium-induced corrosion according to the reaction:V₂O₅ +yY₂O₃ +m(1−y)MgO→2yYVO₄+(1−y)Mg₃V₂O₈+(1−y)(m−3)MgO, wherein y isfrom 0.05 to 0.95, and m is from 3 to
 15. 3. The process of claim 1,wherein the fuel contains sodium.
 4. The process of claim 1, wherein thethermal equipment comprises a gas turbine having a flame temperaturethat is equal to or greater than 1090 degrees Celsius.
 5. The process ofclaim 1, comprising generating the Y₂O₃ from a fat soluble or watersoluble precursor contained in an additive of the yttrium-basedinhibitor.
 6. The process of claim 5, wherein the precursor of the Y₂O₃comprises an inorganic yttrium salt, yttrium sulfonate, an yttriumcarboxylate, an yttrium chloride, or a nanometric yttrium compound insuspension in a hydrophilic or lipophilic solvent.
 7. The process ofclaim 1, comprising generating the MgO from a fat soluble or watersoluble precursor contained in an additive of the magnesium-basedinhibitor.
 8. The process of claim 7, wherein the precursor of the MgOcomprises an inorganic magnesium salt, a magnesium sulfonate, amagnesium carboxylate, or a nanometric magnesium compound in suspensionin a hydrophilic or lipophilic solvent.
 9. The process of claim 1,wherein the introducing of the vanadium corrosion inhibitors into thefluid flow path of the thermal equipment comprises introducing one orboth of the yttrium-based inhibitor and the magnesium-based inhibitorinto one of a plurality of components along a fuel flow path of thethermal equipment, wherein the plurality of components comprises a fueltank, a furnace, and fuel supply circuits upstream of the furnace. 10.The process of claim 9, wherein the introducing of the vanadiumcorrosion inhibitors into the fluid flow path of the thermal equipmentcomprises introducing the yttrium-based inhibitor into a first componentof the plurality of components and introducing the magnesium-basedinhibitor into a second component of the plurality of components, thesecond component being different from the first component.
 11. Theprocess of claim 1, wherein the introducing of the vanadium corrosioninhibitors into the fluid flow path of the thermal equipment comprisesinjecting one or both of the yttrium-based inhibitor and themagnesium-based inhibitor into a water supply circuit or an atomizingair circuit of a furnace of the thermal equipment.
 12. The process ofclaim 1, wherein the introducing of the vanadium corrosion inhibitorsinto the fluid flow path of the thermal equipment comprises injectingone or both of the yttrium-based inhibitor and the magnesium-basedinhibitor into a furnace of the thermal equipment via a nozzle, whereinthe nozzle is a dedicated nozzle or is configured to inject particlesdesigned for cleaning a turbine of the thermal equipment.
 13. Theprocess of claim 2, wherein y is at least from 0.9 to 0.95.
 14. Theprocess of claim 2, wherein y is at most 0.1.